guideline

Handle all exceptions, both user and predefined .

For every exception that might be raised, provide a handler in suitable frames to protect against undesired propagation outside the abstraction .

The statement that "it can never happen" is not an acceptable programming approach. You must assume it can happen and be in control when it does. You should provide defensive code routines for the "cannot get here" conditions.

Some existing advice calls for catching and propagating any exception to the calling unit. This advice can stop a program. You should catch the exception and propagate it or a substitute only if your handler is at the wrong abstraction level to effect recovery. Effecting recovery can be difficult, but the alternative is a program that does not meet its specification.

Making an explicit request for termination implies that your code is in control of the situation and has determined that to be the only safe course of action. Being in control affords opportunities to shut down in a controlled manner (clean up loose ends, close files, release surfaces to manual control, sound alarms) and implies that all available programmed attempts at recovery have been made.